Viruses have a unique ability to enter cells at high efficiency. After entry into cells, viral genes are expressed and the virus replicates. This usually results in the death of the infected cell and the release of the antigenic components of the cell as the cell ruptures as it dies. As a result, virus mediated cell death tends to result in an immune response to these cellular components, including both those derived from the host cell and those encoded by or incorporated into the virus itself. The immune response is also enhanced due to the recognition by the host of so called damage associated molecular patterns (DAMPs) which aid in the activation of the immune response.
Viruses also engage with various mediators of the innate immune response as part of the host response to the recognition of a viral infection through, for example, toll-like receptors, cGAS/STING signalling and the recognition of pathogen associated molecular patterns (PAMPs) resulting in the activation of interferon responses and inflammation which are also immunogenic signals to the host. These immune responses may result in the immunogenic benefit to cancer patients such that immune responses to tumor antigens provide a systemic overall benefit resulting in the treatment of tumors which have not been infected with the virus, including micro-metastatic disease, and providing vaccination against relapse.
The combined direct (‘oncolytic’) effects of the virus, and immune responses against tumor antigens (including non-self ‘neo-antigens’, i.e. derived from the particular mutated genes in individual tumors) is termed ‘oncolytic immunotherapy’.
Viruses may also be used as delivery vehicles (‘vectors’) to express heterologous genes inserted into the viral genome in infected cells. These properties make viruses useful for a variety of biotechnology and medical applications. For example, viruses expressing heterologous therapeutic genes may be used for gene therapy. In the context of oncolytic immunotherapy, delivered genes may include those encoding specific tumor antigens, genes intended to induce immune responses or increase the immunogenicity of antigens released following virus replication and cell death, genes intended to shape the immune response which is generated, genes to increase the general immune activation status of the tumor, or genes to increase the direct oncolytic properties (i.e. cytotoxic effects) of the virus. Importantly, viruses have the ability to deliver encoded molecules which are intended to help to initiate, enhance or shape the systemic anti-tumor immune response directly and selectively to tumors, which may have benefits of e.g. reduced toxicity or of focusing beneficial effects on tumors (including those not infected by the virus) rather than off-target effects on normal (i.e. non-cancerous) tissues as compared to the systemic administration of these same molecules or systemic administration of other molecules targeting the same pathways.
It has been demonstrated that a number of viruses including, for example, herpes simplex virus (HSV) have utility in the oncolytic treatment of cancer. HSV for use in the oncolytic treatment of cancer must be disabled such that it is no longer pathogenic, but can still enter into and kill tumor cells. A number of disabling mutations to HSV, including disruption of the genes encoding ICP34.5, ICP6, and/or thymidine kinase, have been identified which do not prevent the virus from replicating in culture or in tumor tissue in vivo, but which prevent significant replication in normal tissue. HSVs in which only the ICP34.5 genes have been disrupted replicate in many tumor cell types in vitro, and replicate selectively in tumor tissue, but not in surrounding tissue, in mouse tumor models. Clinical trials of ICP34.5 deleted, or ICP34.5 and ICP6 deleted, HSV have also shown safety and selective replication in tumor tissue in humans.
As discussed above, an oncolytic virus, including HSV, may also be used to deliver a therapeutic gene in the treatment of cancer. An ICP34.5 deleted virus of this type additionally deleted for ICP47 and encoding a heterologous gene for GM-CSF has also been tested in clinical trials, including a phase 3 trial in melanoma in which safety and efficacy in man was shown. The trial data demonstrated that tumor responses could be seen in injected tumors, and to a lesser extent in uninjected tumors. Responses tended to be highly durable (months-years), and a survival benefit appeared to be achieved in responding patients. Each of these indicated engagement of the immune system in the treatment of cancer in addition to the direct oncolytic effect. However, this and other data with oncolytic viruses generally showed that not all tumors respond to treatment and not all patients achieve a survival advantage. As a result, improvements to the art of oncolytic therapy are clearly needed. These may serve to increase the direct oncolytic effects of therapy, the anti-tumor immune stimulating effects of the therapy, or both of these effects together.
Recently it has been shown that oncolytic immunotherapy can result in additive or synergistic therapeutic effects in conjunction with immune checkpoint blockade (i.e. inhibition or ‘antagonism’ of immune checkpoint pathways), also referred to as immune co-inhibitory pathway blockade. Checkpoint (immune inhibitory pathway) blockade is intended to block host immune inhibitory mechanisms which usually serve to prevent the occurrence of auto-immunity. However, in cancer patients these mechanisms can also serve to inhibit the induction of or block the potentially beneficial effects of any immune responses induced to tumors. Alternatively, immune responses may not be fully potentiated due to a lack of activation or lack of full activation of immune potentiating pathways. Therefore, drugs which alleviate these blocks (inhibit “immune co-inhibitory pathways”) or stimulate immune potentiating pathways (i.e. which activate, or are ‘agonists’ of “immune co-stimulatory pathways”) are attractive for testing and developing cancer treatments. Targets for such approved or experimental drugs include CTLA-4, PD-1, PD-L1, LAG-3, TIM-3, VISTA, CSF1R, IDO, CEACAM1, GITR, 4-1-BB, KIR, SLAMF7, OX40, CD40, ICOS or CD47.
For many of these approaches targeting immune co-inhibitory or co-inhibitory pathways to be successful, pre-existing immune responses to tumors are needed, i.e. so that a pre-existing immune response can be potentiated or a block to an anti-tumor immune response can be relieved. The presence of an inflamed tumor micro-environment, which is indicative of such an ongoing response, is also needed. Pre-existing immune responses to tumor neo-antigens appear to be particularly important for the activity of immune co-inhibitory pathway blockade and related drugs. Only some patients may have an ongoing immune response to tumor antigens including neoantigens and/or an inflamed tumor microenvironment, both of which are required for the optimal activity of these drugs. Therefore, oncolytic agents which can induce immune responses to tumor antigens, including neoantigens, and/or which can induce an inflamed tumor microenvironment are attractive for use in combination with immune co-inhibitory pathway blockade and immune potentiating drugs. This likely explains the promising combined anti-tumor effects of oncolytic agents and immune co-inhibitory pathway blockade in mice and humans that have so far been observed.
The indoleamine 2,3-dioxygenase (IDO) pathway contributes to tumor-induced tolerance by creating a tolerogenic environment in the tumor and the tumor-draining lymph nodes, both by direct suppression of T cells and enhancement of local regulatory T cell (Treg)-mediated immunosuppression. IDO catalyses the rate-limiting step of tryptophan degradation along the kynurenine pathway, and both the reduction in local tryptophan concentration and the production of immunomodulatory tryptophan metabolites contribute to the immunosuppressive effects of IDO. IDO is chronically activated in many cancer patients with IDO activation correlating with more extensive disease. It can also function as an antagonist to other activators of antitumor immunity. Therefore, inhibitors of the IDO pathway are being developed as anticancer agents, particularly in combination with checkpoint blockade agents such as those which target CTLA-4, PD-1 or PDL-1.
The above discussion demonstrates that there is still much scope for improving oncolytic agents and cancer therapies utilising oncolytic agents.